An extensive set of in situ water vapor (H2O) data obtained by the IAGOS-CARIBIC passenger aircraft at 10–12 km altitude over eight years (2005 – 2013) is analyzed. A multifaceted description of the vertical distribution of H2O from the upper troposphere (UT) via the extra-tropical tropopause mixing layer (exTL) into the lowermost stratosphere (LMS) is given. Compared to longer-lived trace gases, H2O is highly variable in the UT and exTL. It undergoes considerable seasonal variation, with maxima in summer and in phase from the UT up to ~4 km above the tropopause. The transport and dehydration pathways of air starting at the Earth's surface and ending at 10–12 km altitude are reconstructed based upon (i) potential temperature (θ), (ii) relative humidity with respect to ice (RHi), and (iii) back trajectories as a function of altitude relative to the tropopause. RHi [relative humidity with respect to ice] of an air mass was found to be primarily determined by its temperature change during recent vertical movement, i.e. cooling during ascent/expansion and warming during descent/compression. The data show with great clarity that H2O and RHi at 10–12 km altitude are controlled by three dominant transport/dehydration pathways: (i) the Hadley circulation, i.e. convective uplift in the tropics and pole-ward directed subsidence drying from the tropical tropopause layer (TTL) with observed RHi down to 2%, (ii) warm conveyor belts and mid-latitude convection transporting moist air into the UT with observed RHi usually above 60%, and (iii) the Brewer-Dobson shallow and deep branches with observed RHi down to 1%.

8 comments:

Rising air cools and descending air warms. Both without addition or removal of energy. Instead, the form of energy changes from kinetic (heat) to gravitational potential energy (not heat) and back again.

The cause is gas density and weight variations in the horizontal plane which is in turn caused by uneven surface heating.

What so many miss is that the warming of air on the descent phase helps to reduce the rate of surface cooling and so raises the average surface temperature above that expected from the purely radiative S-B expectation. In the case of Earth by 33C.

That surface temperature rise is a consequence of mechanical adiabatic overturning and not downward IR.

Some say that because the process nets out to zero there is no effect on surface temperature but that is wrong for a scenario where there is a constant flow of new (solar) energy through the system.

What happens is that because solar energy arrives constantly the first ascent of the very first convective cycle fails to cool the surface. Instead it reduces the energy lost to space by converting it to gravitational potential energy during uplift. GPE does not radiate.

Then, once one completes the first descent of the very first convective cycle the surface continues to receive ongoing insolation but in addition it is receiving energy from that descending air so the surface temperature must then rise.

That additional surface energy is not availablre for radiation to space because it is immediately taken up in the next convective ascent.

Meanwhile, new insolation continues at the same rate as before so the surface temperature cannot drop back again.

The system equilibriates at a given strength of convective overturning and NOT at a given level of radiative flux though an increased radiative flux within the atmosphere is indeed an inevitable consequence of the mechanically enhanced surface temperature.

GHGs then cause more radiation to space from within the atmosphere but that reduces the energy returning to the surface on descent so convective overturning is weakened by exactly as much as the GHGs radiate to space.

The reduction in energy returning to the surface reduces radiation from the surface to space by exactly as much as the increase in radiation ftrom the atmosphere to space.

It is a self cancelling process at the expense of a miniscule circulation change.

The more of the incoming solar radiative energy that is absorbed by the mass of the atmosphere by conduction the more convection there will be and the more kinetic energy will be converted to gravitational potential energy until the air descends again.

That is why density at the surface ios so important. A denser gas at the surface will divert a larger proportion of incoming solar energy to conduction and convection so the surface temperature rises more for a denser atmosphere at the same level of incoming solar radiation.

It follows that water vapour being less dense than air, an atmosphere composed entirely of water vapour would absorb less energy from the surface by conduction and so the surface would be warmer but the atmosphere cooler than for an atmosphere of Oxygen and Nitrogen.

Not only does water vapour, being less dense absorb less energy from the surface but additionally as a greenhouse gas it radiates to space and further enhances radiation to space from its condensate at a higher level.

Therefore, water vapour has a powerful cooling effect on the surface which is why the moist lapse rate in the ascent is much less than the dry lapse rate on the subsequent descent. Being lighter than air,it converts surface kinetic energy to gravitational potential energy in uplift more effectively than heavier air.

The heavier dry air on the descent reconverts GPE back to kinetic energy more effectively than would lighter water vapour, hence the steeper lapse rate gradient on the descent.

Nonetheless the overall rate of adiabatic convection adjusts in the manner I described so as to rebalance the ratio of radiation to space from surface and atmosphere and thereby ensure that, over time, radiation out to space equals radiation in from space.

For a non radiative atmosphere all energy lossd to space would be from the surface.

For a 100% radiatively effective atmosphere all energy lost to space would depart from within the atmosphere and none from the surface.

Ground avarage temperature is a direct consequence of (*) water condensation temperature zone where massive amount of heat can be absorbed / released, let's say around 20-30°C. Far below, the amount of saturated water vapour is unsignificant and (**) the mass of atmosphere itself. Consider planet Venus, its distance from Sun make theoretical black body equilibrium to ba at 326°K (55°C) and ground temperature is 480°C !!Yes, but its atmosphere is 90 times heavier.... that's the reason for.

Yes, you're right. Even though Earth & Venus have similar mass/gravity, the thick atmosphere on Venus simply means that there’s much more atmospheric mass and more atmospheric pressure at the surface, and therefore from thermodynamic physics alone, such as the adiabatic lapse rate, we expect much higher temperatures at the surface. It also explains why Venus surface temperature does not cool at all during it's 2000 hour long night.

see also: http://hockeyschtick.blogspot.com/2011/08/professor-inadvertently-explains-why.html

same is true for Uranus [and all planets with thick atmospheres] as I mention in this quiz:

The base of the troposphere on the planet Uranus is 320K, considerably hotter than on Earth [288K], despite being nearly 30 times further from the Sun. This is due to

a. Greenhouse gasesb. Pressurec. SUVs

20. If the Earth's atmosphere was 100% nitrogen [instead of 78% presently], what would the base of the troposphere be compared to the equilibrium temperature with the Sun [255K]?

a. Warmerb. Colderc. The same temperature as the equilibrium temperature with the Sun [255K]

21. The base of the troposphere on Uranus is 320K at 100 bars pressure, despite the planet only receiving 3.71 W/m2 energy from the Sun. By the Stefan-Boltzmann Law, a 320K blackbody radiates 584.6 W/m2. Greenhouse gases amplify the energy from the Sun 157.5 times by:

a. Violating the 1st Law of Thermodynamics b. The temperature at the base of the troposphere is due to the ideal gas law PV=nRT, where pressure from gravity and atmospheric mass raise the temperature at the base of the troposphere from the equilibrium temperature with the Sun of 89.94K to 320K, regardless of the atmospheric mixture of greenhouse gases